In current releases of Third Generation Partnership Project (3GPP) Long Term Evolution (LTE), cell-specific reference symbols (CRS) may be used to assist in downlink channel estimation. CRS may also be used for mobility measurements and for uplink power control performed by User Equipment (UE). CRS were designed to be used both for demodulation and channel state information (CSI) feedback purposes. CRS exist in all subframes and are transmitted according to a fixed pattern. Release 10 of LTE introduced a reference signal specifically for CSI purposes referred to as CSI-RS. By measuring on a CSI-RS reference signal, a UE may estimate an effective channel the reference signal is traversing. The estimation may include the radio propagation channel, antenna gains, and possible antenna virtualizations. A CSI-RS port may be precoded so that it is virtualized over multiple physical antenna ports. That is, the CSI-RS port may be transmitted on multiple physical antenna ports, possibly with different gains and phases. LTE also supports UE-specific reference signals designed for assisting in channel estimation for demodulation purposes.
CSI-RS may not be transmitted in every subframe and CSI-RS are generally sparser in time and frequency than reference signals used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to an RRC configured periodicity parameter and an RRC configured subframe offset.
A base station may request a UE operating in connected mode to perform channel state information (CSI) reporting based on received CSI-RS. Such reporting may include a suitable rank indicator (RI), one or more precoding matrix indices (PMIs), and/or a channel quality indicator (CQI). Other types of CSI may include explicit channel feedback and interference covariance feedback. The CSI feedback assists the base station in scheduling. For example, the CSI helps the base station to determine a subframe and resource blocks for a transmission and to determine which transmission scheme and precoder to use. The CSI also provides information on a proper user bit rate for a transmission (link adaptation).
In LTE, both periodic and a-periodic CSI reporting may be supported. For periodic CSI reporting, a UE reports the CSI measurements at a configured, periodic time on the physical uplink control channel (PUCCH). For a-periodic reporting, the CSI feedback is transmitted on the physical uplink shared channel (PUSCH) at a pre-specified time instant after receiving a CSI trigger (e.g., as part of the uplink grant) from a base station. Using a-periodic CSI reports, a base station may dynamically request CSI that reflects downlink radio conditions in a particular subframe.
The RI corresponds to a UE selected number of streams that are to be spatially multiplexed and transmitted in parallel over an effective channel. The PMI identifies a UE selected precoder (in a codebook) for the transmission. The precoder corresponds to a spatial transmission property. The CQI represents a UE selected highest transport format given block error rate (BLER) typically no higher than 10% on the CSI reference resource. Thus, there is a relation between a CQI and a signal to interference plus noise ratio (SINR) of the spatial stream(s) over which the transport block is transmitted.
Reference signals may occupy particular pairs of adjacent time-frequency resource elements within a resource block pair. A pair of adjacent time-frequency resource elements may be referred to as an antenna port reference signal resource (APRSR). The particular APRSRs comprising the reference signal may be referred to as a reference signal configuration. For example, a two-antenna-port CSI-RS configuration may comprise one APRSR. A four-antenna-port CSI-RS configuration may comprise two APRSR. Multiple reference signal resource configurations may form a particular pattern within a particular subframe. In general, time-frequency resource elements associated with a reference signal may be referred to as reference signal resources.
A particular type of CSI-RS may be referred to as zero-power (ZP) CSI-RS (also referred to as muted resource elements) in which no PDSCH signal is transmitted on the CSI-RS resources. ZP-CSI-RS corresponds to a CSI-RS pattern whose time-frequency resource elements are silent. The PDSCH is thus effectively mapped around the ZP-CSI-RS, and the network is free to transmit any signal on the ZP-CSI-RS. Such silent patterns are configured with a smallest resolution of four time-frequency resource elements.
One purpose of ZP-CSI-RS is to raise the SINR for CSI-RS in a cell by configuring ZP-CSI-RS in interfering cells so that time-frequency resource elements that would normally cause interference are silent. A CSI-RS pattern in a particular cell may be matched with a corresponding ZP-CSI-RS pattern in an interfering cell. Raising the SINR level for CSI-RS measurements may be beneficial in applications such as coordinated multi point (CoMP) or in heterogeneous deployments. In joint transmission CoMP, a UE will likely need to measure a channel from non-serving cells, and interference from the much stronger serving cell would be destructive. ZP-CSI-RS may also be used in heterogeneous deployments where ZP-CSI-RS is, configured in a macro-layer so that it coincides with CSI-RS transmissions in a pico-layer. This may avoid strong interference from macro nodes when UEs measure the channel to a pico node.
The PDSCH is mapped around the resource elements occupied by CSI-RS and ZP-CSI-RS so both the network and the UE should be aware of the CSI-RS/ZP-CSI-RS configuration or else the UE may not be able to decode the PDSCH in subframes containing CSI-RS or their ZP counterparts.
LTE Release 11 introduced CSI interference measurement (IM) functionality where the network is able to configure a UE to measure interference on a particular set of resource elements. The network may control the interference seen on a CSI-IM, for example, by muting all transmission points within a coordination cluster on the CSI-IM. In such a scenario, the UE will effectively measure the inter CoMP cluster interference. In this manner, an eNodeB may evaluate the performance of a UE given different CoMP transmission hypotheses. The system may also track or estimate different intra-cluster interference levels corresponding to different transmission and blanking hypotheses. By configuring resources that a UE is using for measuring interference plus noise (e.g., a CSI-IM), a UE may assume that no transmission points of interest are transmitting on that resource, and the received power may be used as a measure of the interference plus noise.
In uncoordinated systems, a UE may measure the interference observed from other transmission points (or other cells). The measured interference corresponds to a relevant interference level in an upcoming data transmission.
In coordinated systems, the network may control which transmission points that may possibly interfere with a UE. Multiple interference hypotheses may exist depending on which transmission points are transmitting data to other UEs. The network may also choose to transmit interference from specific transmission points for the sole purpose of testing how that particular interference hypothesis affects a UE (as seen via the CSI feedback). The network may also asses which hypothesis is most likely to occur for upcoming transmission time intervals. This may be referred to as a traffic hypothesis. For UEs that are on the boundaries of a coordination area, the dominant interferers may not be found within the coordinated transmission points but instead from outside the cluster. This inter-cluster interference (ICI) may not be controlled and may impact CSI reports in an unknown way.
In LTE Release 11, CSI processes are defined such that each CSI process is associated with a CSI-RS resource configuration and a CSI-IM resource configuration. The network may configure a UE in transmission mode 10 with one to four CSI processes. A CSI reported by the UE is associated with a particular CSI process. The network may test different interference scenarios simultaneously on the UE using the four CSI (spanning maximally three different CSI-IMs). Based on the UE report feedback, the network may adopt a particular transmission scheme.